Active transport of protons across cellular membranes is a key feature of biological energy transduction processes. Proton translocation is well characterized in energetic terms: in chloroplasts the energy derived from light generates a transmembrane proton gradient which is coupled to ATP synthesis by reversal of another proton transport system; in mitochondria energy derived from oxidative processes drives a similar set of coupled proton transporters. The best characterized proton translocation system is the purple membrane of Halobacterium halobium. This structure is a crystalline array containing only one polypeptide species, bacteriorhodopsin. This protein is a light-driven proton pump. Bacteriorhodopsin photoreaction intermediates have been defined spectroscopically, and the three-dimensional topology of this protein in the purple membrane lattice has been well described. The relationship of function to structure is, however, not understood. I propose to approach this problem by studying variant bacteriorhodopsin molecules with altered function. A direct way to obtain such variants is by selecting H. halobium mutant strains with altered bacteriorhodopsin. Until the recent identification of bacteriorhodopsin as a receptor for phototaxis, the absence of a selection procedure has precluded this genetic approach. By using the sensory function of bacteriorhodopsin, we will isolate mutant halobacteria with specific alterations in the protein. Variant bacteriorhodopsin from these strains will be examined with biochemical, immunological, and spectroscopic methods.